The present invention relates to a reactive power control cycloconverter which optionally controls, in accordance with specific instructions, the power factor of a fundamental wave at the power supply side.
A cycloconverter of a power supply system directly converts AC power having a given frequency to that having a different frequency. Thyristors used as the components of the cycloconverter are commutated by a power supply voltage, so a prominent amount of reactive power must be supplied from a power supply. Further, the reactive power continuously varies in synchronism with the frequency of a load. From this, in addition to a disadvantage that a large capacity of power supply must be provided, various electrical devices connected to the same power supply system are adversely affected by changes in reactive power.
A reactive power compensator is conventionally coupled to the input side of a cycloconverter in order to compensate for the changes in reactive power at the input side of the cycloconverter. Such a reactive power compensator must have quick response characteristics so as to fully compensate for the changes in reactive power. For this purpose, semiconductor elements such as thyristors are often used, but the use of such elements results in high manufacturing cost.
FIG. 1 shows a reactive power control cycloconverter, which is a background art of the present invention. Reference symbol CC denotes the main unit of a circulating current type cycloconverter; SS-P and SS-N denote positive and negative converters, respectively; LO1 and LO2 respectively denote DC reactors with center taps; and LOAD denotes a load of the cycloconverter. Reference symbol TR denotes a power transformer; C denotes a delta- or Y-connected phase advancing capacitor; and BUS denotes three-phase power lines. A control circuit for the cycloconverter includes a current transformer CTS for detecting three-phase AC currents at the input side of CC, a voltage transformer PT for detecting three-phase AC voltages, a reactive power arithmetic circuit VAR, a control compensator H(S), a current transformer CTP for detecting an output current IP from positive converter SS-P, a current transformer CTN for detecting an output current IN from negative converter SS-N, adders A1 to A5, operational amplifiers K0 to K3, comparators C1 to C3, an absolute value circuit ABS and phase control circuits PH-P and PH-N.
Current IL=(IP-IN) is obtained from adder A3. Current IL represents the detected value of the load current. The following calculation is performed by the combination of adder A1, adder A2, absolute value circuit ABS and amplifier K0=(1/2): EQU Io=(IP+IN-.vertline.IL.vertline.)/2 (1)
Current Io is a detected value of the circulating current.
The load current control operation of FIG. 1 is as follows.
Specified load current instruction IL* is compared with the detected value IL which indicates the actual load current. Phase control circuits PH-P and PH-N are controlled such that the cycloconverter generates a voltage in proportion to the difference .epsilon.3 between currents IL* and IL. Phase control circuit PH-N receives an input from amplifier K2 through inverting amplifier K3, so that the output phase .alpha.N of phase control circuit PH-N is set to be 180.degree.-.alpha.P, where .alpha.P denotes the output phase of phase control circuit PH-P.
The normal operation of FIG. 1 is that the output voltage VP of positive converter SS-P is balanced, at the load side of the cycloconverter, with the output voltage VN of negative converter SS-N, as follows: ##EQU1## where VS is the power supply voltage and kv is the proportional constant. When load current instruction IL* sinusoidally changes, difference .epsilon.3 also changes so that output phases .alpha.P and .alpha.N are controlled in a specific manner, thereby flowing through the load a sinusoidal current IL. In this normal operation, the output voltage of positive converter SS-P is well-balanced with the output voltage of negative converter SS-N, and no circulating current Io flows.
The operation of the circulating current control is as follows.
Current transformer CTS and voltage transformer PT are arranged at the power supply side (input side of the cycloconverter). Reactive power Q is calculated in reactive power arithmetic circuit VAR. Specified value (specific instruction) Q* for the reactive power is normally set to be zero. Comparator C1 provides difference .epsilon.1 (=Q*-Q). Control compensator H(S) includes an integration element for nullifying the steady difference .epsilon.1. An output Io* from control compensator H(S) becomes the specified value (specific instruction) of a circulating current Io. Comparator C2 generates difference .epsilon.2=(Io*-Io) which is supplied to adders A4 and A5 through amplifier K1.
Inputs .epsilon.4 and .epsilon.5 respectively supplied to phase control circuits PH-P and PH-N are given, under the assumption that K3=-1, as follows: EQU .epsilon.4=K2.multidot..epsilon.3+K1.multidot..epsilon.2 (4) EQU .epsilon.5=-K2.multidot..epsilon.3+K1.multidot..epsilon.2 (5)
Then, relation .alpha.N=180.degree.-.alpha.P cannot be maintained any longer, so output voltage VP of positive converter SS-P comes to be unbalanced, by an amount being proportional to K1.multidot..epsilon.2, from output voltage VN of negative converter SS-N. The voltage difference [VP-VN] is applied to DC reactors LO1 and LO2, so that circulating current Io flows. When current Io exceeds the specified value (instruction) Io, difference .epsilon.2 is reduced so as to decrease the difference [VP-VN]. As a result, current Io is controlled so that Io coincides with Io*.
When reactive power Q is advancing in phase, difference .epsilon.1=Q*-Q=-Q becomes positive. Then, specified value Io* is increased, and a phase-delayed reactive current of the cycloconverter increases. The circulating current Io is so controlled that the condition Q=Q*=(0) is finally obtained. When reactive power Q is delayed in phase, inequality .epsilon.1&lt;0 is established to decrease the value Io*, so that current Io is controlled to establish the relation Q=0. In this manner, the reactive power at the input side of the cycloconverter becomes zero. In other words, the power factor of a fundamental wave at the power supply side can be held "1".
FIG. 2 shows voltage-current vectors at the input side of the cycloconverter in FIG. 1. Reference numeral Vs denotes the power supply voltage; Icap denotes the current of phase advancing capacitor C; ISSP denotes the input current of positive converter SS-P; ISSN denotes the input current of negative converter SS-N; Icc denotes the input current of the cycloconverter; Ireact denotes the reactive component of input current Icc; and Is denotes the power supply current. This vector diagram illustrates the magnitudes and phase angles of respective signal components relating to the load current at a certain instant, and the load current spontaneously changes. The values of currents ISSP and ISSN and phase angles .alpha.P and .alpha.N also spontaneously change.
When the reactive power control is performed (Q*=0), circulating current Io is controlled to establish the relation Icap=Ireact. Reactive current component Ireact is given, under the assumption .alpha.N=180.degree.-.alpha.P, as follows: ##EQU2## where k1 is the conversion constant of the cycloconverter. When the operation of control is performed to establish the relation Q=0 or Icap=Ireact, the circulating current Io satisfies the following equation: EQU Io=(Icap-k1.multidot..vertline.IL.vertline..multidot.sin .alpha.P)/(2k1.multidot.sin .alpha.P) (7)
Even in the background art reactive power control cycloconverter, the fundamental wave power factor at the input side can be fixed at "1" without using an external reactive power compensator. However, in order to control the reactive power, a circulating current is necessary. For this purpose, a pair of positive and negative converters SS-P and SS-N must be provided. Thus, when the background art cycloconverter of this type is employed for a three-phase induction motor or a three-phase synchronous motor, at least six AC-DC power converters must be provided, so that the main control circuit and its related circuit arrangement of the cycloconverter become complex, resulting in high cost and low reliability.